We have accidentally left MR12 in the tank for as long as two weeks without any fuel pump damage or gumming of the fuel system – we could NOT say the same about MR9. Of course, this is not a good idea and one other item of consideration is that the gas tank is vented – this means that the oxygenates, which create the additional power, can evaporate quickly if left in the tank. We have seen measurable power losses if left in the tank for over two days.
Update: we intentionally left MR12 in the fuel tank of our 2017 GSX-R1000 for eight weeks, with no ill effects. Our estimate is that Euro 4 compliant machines have more robust fuel system components. Of course, this is not recommended.